The present invention relates to an arrival direction estimation method using an array antenna for estimating an arrival direction of a desired signal. The present invention further relates to a DS-CDMA receiver unit in a Direct Sequence Code Division Multiple Access (DS-CDMA) system.
In a base station of a DS-CDMA communication system, signal reception is performed by utilizing an array antenna. Further, an arrival direction of a desired signal is estimated from signals received from the array antenna. By transmitting and receiving in a beam directed in the estimated arrival direction, it is possible to reduce both interference and required transmission power due to an increase in antenna gain. Therefore, it is desirable that the arrival direction of the desired signal is estimated accurately.
FIG. 7 is a diagram showing a receiver unit of a base station using an array antenna. In particular, FIG. 7 shows array antenna elements 71-0 through 71-(Mxe2x88x921) forming a receiving antenna, reception processors 72-0 to 72-(Kxe2x88x921), despreading units 73-0 to 73-(Mxe2x88x921), beam forming unit 74, a channel receiver 75, an arrival direction estimation unit 76 and a beam former 77.
Received signals r(0) to r(Mxe2x88x921) from the array antenna elements 71-0 to 71-(Mxe2x88x921) are input to the reception processors 72-0 through 72-(Kxe2x88x921) corresponding to the different channels. In addition, the reception processor 72-0 carries out a despreading process with respect to the received signals by the despreading units 73-0 to 73-(Mxe2x88x921), which are input with a spreading code C0. The despreading units 73-0 to 73-(Mxe2x88x921) provide despread output signals x(o) to x(Mxe2x88x921) to the arrival direction estimation unit 76 and the beam former 77 of the beam forming unit 74. The arrival direction estimation unit 76 estimates an arrival direction of a desired signal based on a cross-correlation function among the received signals received by adjacent antenna elements.
The beam former 77 multiplies a weighting coefficient obtained by estimating the arrival direction by the arrival direction estimation unit 76 to the despreading output signals x(0) through x(Mxe2x88x921). Further, the beam former 77 then combines the products to provide an output signal to the channel receiver 75.Therefore, received data of the kth channel is re-generated by the reception processors 72-0 to 72-(Kxe2x88x921) corresponding to channels 0 through Kxe2x88x921.
FIG. 8 is a diagram showing a conventional beam forming unit 74 including the arrival direction estimation unit 76. In particular, FIG. 8 shows the beam forming unit 74 including an arrival direction estimation unit 76 and a beam former 77 having multipliers 78-0 to 78-3 and an adder 79.
Received signals x(0)(n) to x(3)(n) from the antenna elements 71-0 to 71-3 (refer to FIG. 7) are input to the arrival direction estimation unit 76 and the beam former 77. The arrival direction estimation unit 76 outputs to the beam former 77 weighting coefficients a0 to a3 obtained by the estimation of the arrival direction of the desired signal. The weighting coefficients a0 to a3 are respectively input to the multipliers 78-0 to 78-3 of the beam former 77 to be multiplied with signals x(0)(n) to x(3)(n) and then coherently summed by the adder 79. An output signal y(n) of the adder 79 is then input to the channel receiver 75 (refer to FIG. 7).
FIG. 9 is a diagram showing a conventional arrival direction estimation unit. In particular, FIG. 9 shows an arrival direction estimation unit 76 including correlation calculators 81-0 through 81-2, an adder 82, and an array weighting coefficient calculator 83. A correlation calculator 81 is shown in the lower part of FIG. 9, wherein each of the correlation calculators 81-0 to 81-2 include a multiplier 84 and an averaging filter 85. Further, received signals x(m)(n) and x(m+1)(n) from mth and (m+1)th adjacent antenna elements are input to the multiplier 84. In this case, one of the received signals is a complex conjugate (indicated by *), and a complex conjugate product between the received signals from the adjacent antenna elements (cross-correlation function of the zero lag) is obtained. A temporal average is also performed by the averaging filter 85 so that a cross-correlation function R(m) is output.
A received signal r(m) (t) from a mth antenna element at a time t satisfying (nxe2x88x921)Txe2x89xa6t less than nT is described by the following formula (1), where xc3x8i(m) for I=0 to Nxe2x88x921 in formula (1) is described by formula (2), N denotes a number of users, Ai denotes a received amplitude of an ith user signal, ci (t) denotes a spreading code of the ith user, xcfx84i denotes a relative delay of the ith user, bi denotes an ith user transmission symbol, d denotes a distance between the antenna elements, xcex8i denotes an arrival angle of the ith user signal, N(m)(t) denotes a noise signal, T denotes a symbol length, and a multipath is not taken into consideration for the sake of convenience.                                           r                          (              m              )                                ⁢                      xe2x80x83                    ⁢                      (            t            )                          =                                            ∑                              i                =                0                                            N                -                1                                      ⁢                          xe2x80x83                        ⁢                                          A                i                            ⁢                              xe2x80x83                            ⁢                              c                i                            ⁢                              xe2x80x83                            ⁢                              (                                  t                  -                                      τ                    I                                                  )                            ⁢                              xe2x80x83                            ⁢              b              ⁢                              xe2x80x83                            ⁢                              (                                  t                  -                                      τ                    i                                                  )                            ⁢                              xe2x80x83                            ⁢              exp              ⁢                              xe2x80x83                            ⁢                              (                                                      -                    j                                    ⁢                                      xe2x80x83                                    ⁢                                      φ                                          (                      m                      )                                                                      )                                              +                                    N                              (                m                )                                      ⁢                          xe2x80x83                        ⁢                          (              t              )                                                          (        1        )                                          φ          i                      (            m            )                          =                  2          ⁢                      xe2x80x83                    ⁢          π          ⁢                      xe2x80x83                    ⁢          md          ⁢                      xe2x80x83                    ⁢          sin          ⁢                      xe2x80x83                    ⁢                      θ            i                                              (        2        )            
A despreading output signal x(k)(m)(n) which is obtained by subjecting the received signal r(m) (t) from the mth antenna element to a despreading process by the spreading code c(k) (t) of the kth user is described by the following formula (3), where wij(n) in the formula (3) is described by the formula (4).                                                                                           x                  k                                      (                    m                    )                                                  ⁢                                  xe2x80x83                                ⁢                                  (                  n                  )                                            =                              xe2x80x83                            ⁢                                                ∫                                                            (                                              n                        -                        1                                            )                                        ⁢                                          xe2x80x83                                        ⁢                    T                                                        n                    ⁢                                          xe2x80x83                                        ⁢                    T                                                  ⁢                                                      r                                          (                      m                      )                                                        ⁢                                      xe2x80x83                                    ⁢                                      (                    t                    )                                    ⁢                                      xe2x80x83                                    ⁢                                      c                    k                                    *                                      (                                          t                      -                                              τ                        k                                                              )                                    ⁢                                      xe2x80x83                                    ⁢                                      ⅆ                    t                                                                                                                          =                              xe2x80x83                            ⁢                                                                    A                    k                                    ⁢                                      xe2x80x83                                    ⁢                                      b                    k                                    ⁢                                      xe2x80x83                                    ⁢                                      (                    n                    )                                    ⁢                                      xe2x80x83                                    ⁢                  exp                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  -                        j                                            ⁢                                              xe2x80x83                                            ⁢                                              φ                                                  (                          m                          )                                                                                      )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                    ∫                                                                  (                                                  n                          -                          1                                                )                                            ⁢                                              xe2x80x83                                            ⁢                      T                                                              n                      ⁢                                              xe2x80x83                                            ⁢                      T                                                        ⁢                                                            [                                                                        ∑                                                                                    i                              =                              0                                                                                      i                              ≠                              k                                                                                                            N                            -                            1                                                                          ⁢                                                  xe2x80x83                                                ⁢                                                                              A                            i                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      c                            i                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      (                                                          t                              -                                                              τ                                i                                                                                      )                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      c                            k                                                    *                                                      (                                                          t                              -                                                              τ                                k                                                                                      )                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      b                            i                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      (                                                          t                              -                                                              τ                                i                                                                                      )                                                    ⁢                                                      xe2x80x83                                                    ⁢                          exp                          ⁢                                                      xe2x80x83                                                    ⁢                                                      (                                                                                          -                                j                                                            ⁢                                                              xe2x80x83                                                            ⁢                                                              φ                                                                  (                                  m                                  )                                                                                                                      )                                                                                              ]                                        ⁢                                          ⅆ                      t                                                                      +                                                                                                        xe2x80x83                            ⁢                                                N                                      (                    m                    )                                                  ⁢                                  xe2x80x83                                ⁢                                  (                  n                  )                                                                                                        =                              xe2x80x83                            ⁢                                                                    A                    k                                    ⁢                                      xe2x80x83                                    ⁢                                      b                    k                                    ⁢                                      xe2x80x83                                    ⁢                                      (                    n                    )                                    ⁢                                      xe2x80x83                                    ⁢                  exp                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  -                        j                                            ⁢                                              xe2x80x83                                            ⁢                                              φ                                                  (                          m                          )                                                                                      )                                                  +                                                                                                        xe2x80x83                            ⁢                                                                    ∑                                                                  i                        =                        0                                                                    i                        ≠                        k                                                                                    N                      -                      1                                                        ⁢                                                            [                                                                        ∫                                                                                    (                                                              n                                -                                1                                                            )                                                        ⁢                                                          xe2x80x83                                                        ⁢                            T                                                                                n                            ⁢                                                          xe2x80x83                                                        ⁢                            T                                                                          ⁢                                                                              c                            i                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      (                                                          t                              -                                                              τ                                i                                                                                      )                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      c                            k                                                    *                                                      (                                                          t                              -                                                              τ                                k                                                                                      )                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      b                            i                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      (                                                          t                              -                                                              τ                                i                                                                                      )                                                    ⁢                                                      ⅆ                            t                                                                                              ⁢                                              xe2x80x83                                            ]                                        ⁢                    exp                    ⁢                                          xe2x80x83                                        ⁢                                          (                                                                        -                          j                                                ⁢                                                  xe2x80x83                                                ⁢                                                  φ                                                      (                            m                            )                                                                                              )                                                                      +                                                                                                        xe2x80x83                            ⁢                                                N                                      (                    m                    )                                                  ⁢                                  xe2x80x83                                ⁢                                  (                  n                  )                                                                                                        ≅                              xe2x80x83                            ⁢                                                                    A                    k                                    ⁢                                      xe2x80x83                                    ⁢                                      b                    k                                    ⁢                                      xe2x80x83                                    ⁢                                      (                    n                    )                                    ⁢                                      xe2x80x83                                    ⁢                  exp                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  -                        j                                            ⁢                                              xe2x80x83                                            ⁢                                              φ                                                  (                          m                          )                                                                                      )                                                  +                                                      ∑                                                                  i                        =                        0                                                                    i                        ≠                        k                                                                                    N                      -                      1                                                        ⁢                                      xe2x80x83                                    ⁢                                      [                                                                  A                        i                                            ⁢                                              xe2x80x83                                            ⁢                                              w                        ij                                            ⁢                                              xe2x80x83                                            ⁢                                              (                        n                        )                                            ⁢                                              xe2x80x83                                            ⁢                      exp                      ⁢                                              xe2x80x83                                            ⁢                                              (                                                                              -                            j                                                    ⁢                                                      xe2x80x83                                                    ⁢                                                      φ                            i                                                          (                              m                              )                                                                                                      )                                                              ]                                                  +                                                      N                                          (                      m                      )                                                        ⁢                                      xe2x80x83                                    ⁢                                      (                    n                    )                                                                                                          (        3        )                                                                    w              ij                        ⁢                          xe2x80x83                        ⁢                          (              n              )                                ≡                      xe2x80x83                    ⁢                                    ∫                                                (                                      n                    -                    1                                    )                                ⁢                                  xe2x80x83                                ⁢                T                                            n                ⁢                                  xe2x80x83                                ⁢                T                                      ⁢                                          c                I                            ⁢                              xe2x80x83                            ⁢                              (                                  t                  -                                      τ                    i                                                  )                            ⁢                              xe2x80x83                            ⁢                              c                k                            *                              (                                  t                  -                                      τ                    k                                                  )                            ⁢                              xe2x80x83                            ⁢              bi              ⁢                              xe2x80x83                            ⁢                              (                                  t                  -                                      τ                    i                                                  )                            ⁢                              xe2x80x83                            ⁢                              ⅆ                t                                                    ⁢                  xe2x80x83                                    (        4        )            
In addition, the correlation calculator 81 of FIG. 9 calculates a correlation Rk(m) between the received symbol x(m)(n) of the mth antenna element and the received symbol x(m+1)(n) of the (m+1) th antenna element based on the following formula (5), where brackets xe2x80x9c less than   greater than xe2x80x9d indicate averaging and the terms inside the brackets xe2x80x9c less than   greater than xe2x80x9d are shown in the formula (6).                                           R            k                          (              m              )                                =                       less than                                           x                k                                  (                  m                  )                                            ⁢                              xe2x80x83                            ⁢                              (                n                )                            ⁢                              xe2x80x83                            ⁢                              x                k                                                      (                                          m                      +                      1                                        )                                    *                                            ⁢                              xe2x80x83                            ⁢                              (                n                )                                       greater than                           ⁢                  
                                    (        5        )                                                      x            k                          (              m              )                                ⁢                      xe2x80x83                    ⁢                      (            n            )                    ⁢                      xe2x80x83                    ⁢                      x            k                                          (                                  m                  +                  1                                )                            *                                ⁢                      xe2x80x83                    ⁢                      (            n            )                          =                                                            "LeftBracketingBar"                                  A                  k                                "RightBracketingBar"                            2                        ⁢                          xe2x80x83                        ⁢                                          "LeftBracketingBar"                b                "RightBracketingBar"                            2                        ⁢                          xe2x80x83                        ⁢                          exp              ⁡                              [                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  φ                        k                                                  (                          m                          )                                                                    -                                              φ                        k                                                  (                                                      m                            +                            1                                                    )                                                                                      )                                                  ]                                              +                                    (        6        )                                                      ∑                                          i                =                0                                            i                ≠                k                                                    N              -              1                                ⁢                      xe2x80x83                    ⁢                                                    "LeftBracketingBar"                                  A                  i                                "RightBracketingBar"                            2                        ⁢                          xe2x80x83                        ⁢                                          "LeftBracketingBar"                                                      W                    ik                                    ⁢                                      xe2x80x83                                    ⁢                                      (                    n                    )                                                  "RightBracketingBar"                            2                        ⁢                          xe2x80x83                        ⁢                          exp              ⁡                              [                                  j                  ⁢                                      xe2x80x83                                    ⁢                                      (                                                                  φ                        k                                                  (                          m                          )                                                                    -                                              φ                        k                                                  (                                                      m                            +                            1                                                    )                                                                                      )                                                  ]                                                    +                              N                                          (                m                )                            *                                ⁢                      xe2x80x83                    ⁢                      (            n            )                    ⁢                      xe2x80x83                    ⁢                      N                          (              m              )                                *          n                                    xe2x80x83            
The correlation Rk(m) is then averaged for each of the antenna elements and a cross-correlation function Rk among the adjacent antenna elements is obtained based on the following formula (7), where M denotes the number of antenna elements.                               R          k                =                              [                          1              ⁢                              /                            ⁢                              (                                  M                  -                  1                                )                                      ]                    ⁢                                    ∑                              m                =                0                                            M                -                2                                      ⁢                          xe2x80x83                        ⁢                          R              k                              (                m                )                                                                        (        7        )            
FIG. 9 shows a structure for adding the correlations R(0) to R(2) from the correlation calculators 81-0 to 81-2 by the adder 82. However, no calculation function related to the term [1/(Mxe2x88x921)] in the formula (7) is shown. Since (Mxe2x88x921) is a fixed value, it is clear that an averaging process can be carried out based on the addition.
In formula (6) as described above, the second term on the right hand side of the equal sign is an uncorrelated signal and the term |Wik(n)| is a small value. Hence, if there is no significant difference among the received signal amplitudes Ai of each of the users, it is possible to extract by averaging a phase error of a desired signal (kth user) of the first term on the right hand side of the equal sign of the formula (6). Accordingly, based on a ratio of an imaginary part Im(Rk) and a real part Re (Rk)using the cross-correlation function Rk of the formula (7), it is possible to estimate, from the following formula (8), an arrival angle xcex8k of the kth user signal.
xcex8k=sinxe2x88x921[(xc2xdxcfx80d)tanxe2x88x921{Im(Rk)/Re(Rk)}]xe2x80x83xe2x80x83(8)
According to the conventional arrival direction estimation method described above, the arrival direction is estimated from the strongest signal received after the despreading process, regardless of whether or not the desired signal exists. For this reason, it is difficult to estimate the correct arrival direction of the desired signal in cases where the reception amplitude of the desired signal is small and in cases where large interference signals exist.
For example, when a multi-rate transmission including a high-speed rate and a low-speed rate in the DS-CDMA communication system is made, the received signal of the high-speed rate user becomes an interference signal for the received signal of the low-speed rate user since the transmission power of the high-speed rate signal is large. Consequently, a problem exists in that it is difficult to estimate the arrival direction of the desired signal of the low-speed rate user.
Accordingly, it is an object of the present invention to enable estimation of the correct arrival direction of the desired signal in a DS-CDMA communication system using a code having a period longer than the symbol length of the spreading code, even when a interference signal exists.
These and other objects are met in accordance with an arrival direction estimation method of the present invention, using an array antenna in a DS-CDMA communication system for performing a transmission by performing a spreading process including a code having a period longer than a symbol length. The method also includes to subject received signals from the array antenna to a despreading process. A cross-correlation function of lags other than the zero lag for signals corresponding to adjacent antenna elements is obtained by inverse-modulating despead output signals by a known symbol. Further, an arrival direction of a desired signal based on the cross-correlation function is estimated.
It is also possible to use an average value of the cross-correlation function for a plurality of different lags as the cross-correlation function of the lags among the inverse modulated signals corresponding to the adjacent antenna elements, wherein the zero lag is excluded from this function. It is also possible to obtain the cross-correlation function using both positive and negative lags excluding the zero lag among the inverse modulated signals corresponding to the adjacent antenna elements. A pilot symbol may be used as the known symbol to inverse-modulate the received signals. It is also possible to provide a decision feedback symbol as the known symbol to inverse-modulate the received signals.
A DS-CDMA receiver unit according to the present invention is also provided for a DS-CDMA communication system, which carries out a transmission by performing a spreading process using a code having a period longer than a symbol length. The DS-CDMA includes despreading units for subjecting received signals from array antenna elements to a despreading process. Further included is a beam forming unit 4 for inputting despread output signals from the despreading units and a channel receiver for inputting output signals of the beam forming unit. The beam forming unit includes an inverse modulator for inverse-modulating the despread output signals by a known symbol, an arrival direction estimation unit for receiving signals from the inverse modulator and a beam former. The beam former provides an output by multiplying a weighting coefficient from the arrival direction estimation unit to the despread output signals and then adding such products.
The inverse modulator may be configured to use either a pilot symbol or a decision feedback signal, as the known symbol. The arrival direction estimation unit may include a correlation calculator for obtaining a cross-correlation among output signals of the inverse modulator corresponding to the adjacent antenna elements and an array weighting coefficient calculator for obtaining and averaging the cross-correlation functions with respect to a plurality of lags excluding a zero lag. The array weighting coefficient calculator then outputs a weighting coefficient obtained as a result of estimating the arrival direction based thereon. In this case, the cross-correlation function with respect to the plurality of lags may be calculated by providing first and second shift registers.